The present invention generally relates to electric machines and, more specifically, to apparatus and methods for cooling high speed generators using a cooling oil spray directed to the generator rotor coils.
Generator systems found in aircraft, ships and some terrestrial vehicles, may include three separate brushless generators, namely, a permanent magnet generator (PMG), an exciter, and a main generator. The PMG generates AC currents that are converted to a DC current that is provided to stator windings of the exciter. The exciter rotor generates AC current that is rectified to DC current and provided to rotor windings of the main generator, which generates a three phase AC output within the main stator windings that can be provided to a load such as, for example, an aircraft, ship, or vehicle electrical system.
The electrical load requirements for such generator systems are increasing. As a result, larger generators are needed which are capable of producing more power. However as the machines become longer it becomes increasingly difficult to remove heat from the center of the machine. One of the largest single areas of heat generation takes place in the rotor coils, so one prior approach is to apply a cooling oil spray to the rotor coil end turns. This cooling approach creates a temperature gradient between the end turns and the center of the rotor coil. The longer the machine the larger the temperature gradient. Due to the increasing length of modern aerospace generators the technique of cooling only the end turns of the rotor coils will not provide adequate cooling to the central region between the two coil ends. This results in unacceptably high temperatures at the rotor center, which can lead to a breakdown of winding insulation and failure of the machine. Other generator cooling systems rely on the conduction of heat to a cooling fluid passing across the rotor. However, such methods relying on cooling by conduction still do not provide adequate cooling to avoid hot spots near the center to the rotor.
The problem of cooling the center of a generator rotor presents a general limitation on the power density of electrical power generators of many different sizes and power output levels. In order to compensate for the excess heat at the rotor center, many generators must be made larger than would otherwise be required, thereby lowering the power density. However, increasing the size of the generator adds to the overall weight and cost of the electrical generation system.
As can be seen, there is a need for an improved cooling system for electrical generators that more effectively removes heat from all areas of the generator rotor, particularly the rotor central region. There is also a need for such a cooling system that is adaptable to a variety of sizes and types of generators and which can be applied to high speed, high power generator systems. There is a further need for a generator cooling system that will enable generators having greater power density, thereby reducing the cost and weight of the overall generator system.